1. Field of the Invention
The present invention relates generally to improvements in gyroscopes, and more particularly to, hemispherical resonator gyroscopes (HRGs).
2. Description of the Related Art
A hemispherical resonator gyroscope (HRG) is a vibratory sensor that includes a hemispherical resonator. Examples of HRGs can be found in U.S. Pat. Nos. 3,625,067, 3,656,354, 3,678,762, 3,719,074, 4,157,041, 4,951,508, 5,763,780, 5,801,310, 5,892,152, 5,902,930, 5,983,719, 6,065,340, 6,079,270, 6,158,282, 6,565,395, 6,619,121, and 6,883,361, each of which is hereby incorporated, in its entirety, by reference.
A main component of each HRG is a quartz high Q resonator. This resonator is driven into oscillation by application of electrostatic forcers that are synchronized to the individual natural frequency of the resonator. Since each individual resonator includes its own individual natural frequency, the frequency is often referred to as “4.xx kHz”, which is approximately 4.1 kHz for a 30 millimeter diameter resonator. For example, a first resonator may include a natural frequency of 4.07 kHz, while a second resonator includes a natural frequency of 4.13 kHz.
The electrostatic forcers cause the resonator to flex in an elliptical mode of oscillation. A set of readout electrodes arranged around the resonator are used to sense the amplitude, location, and motion of the elliptical mode standing wave pattern (the flex wave) resulting from such oscillation. If the HRG is rotated, the angle of rotation can be determined from processing a signal output by amplifiers attached to the readout electrodes (i.e., by observing changes in the position of the flex wave or actions necessary to prevent changes in the position of the flex wave).
FIG. 1 illustrates an HRG 100 partitioned into four functional segments: a sensor segment 110, a control drive segment 120, a readout segment 130, and a signal processing segment 140. Sensor segment 110 includes a hemispherical resonator 1110, a plurality of drive electrodes 1120, and a plurality of readout electrodes 1130. The plurality of drive electrodes 1120 includes a set of amplitude control drive electrodes 1122, a set of quadrature control drive electrodes 1124, and a set of rate control drive electrodes 1126. The plurality of readout electrodes 1130 includes a set of X-axis readout electrodes 1132, and a set of Y-axis readout electrodes 1134.
Control drive segment 120 is in communication with sensor segment 110 to provide an amplitude control drive signal 1210 to the set of amplitude control drive electrodes 1122, a quadrature control drive signal 1220 to the set of quadrature control drive electrodes 1124, and a rate control drive signal 1230 to the set of rate control drive electrodes 1126. Sensor segment 110 is also in communication readout segment 130 and provides an X readout signal 1310 from X readout electrodes 1132, and a Y readout signal 1320 from Y readout electrodes 1134 to readout segment 130. Readout segment 130 is in communication with signal processing segment 140 to provide signal processing segment 140 with X readout signal 1310 and Y readout signal 1320. X readout signal 1310 is derived from signals provided by X readout electrodes 1132, and Y readout signal 1320 is derived from signals provided by Y readout electrodes 1134. X readout signal 1310 and Y readout signal 1320 are utilized by processing segment 140 to provide an inertial angle output signal 150. X readout signal 1310 and Y readout signal 1320 are also used in processing segment 140 to provide amplitude drive signal 1210, quadrature control drive signal 1220, and rate control drive signal 1230 to control drive unit 120.
Control of HRG 100, and detection of any rotation of HRG 100 are provided by X readout signal 1310 and Y readout signal 1320. Each electrode of X readout electrodes 1132 and Y readout electrodes 1134 provides an electrode readout signal (SER) to readout segment 130. SER is related to a bias voltage (Vb) and to the amplitude (A) of the flex wave, where the relationship can be described using equation (E1) below. In equation (E1), Vb is the bias voltage applied to hemispherical resonator 1110 (which in some instances is metal clad so that its surface is electrically conductive), ωR (≈4.1 kHz) is the natural frequency of hemispherical resonator 1110 of sensor segment 110, A is the amplitude of the flex wave over X readout electrodes 1132 and Y readout electrodes 1134, φ is a phase offset, and Kr is a proportionality constant.SER=Kr*Vb[1−A*Cos(ωRt+φ)]  (E1)
To operate HRG 100, three types of control forces are applied to hemispherical resonator 1110. These forces correspond to amplitude drive signal 1210, quadrature control drive signal 1220, and rate control drive signal 1230 provided by control drive segment 120. Amplitude drive signal 1210 is used to provide amplitude control of the flex wave and to keep hemispherical resonator 1110 oscillating at or near its natural (resonant) frequency. Quadrature control drive signal 1220 is used to suppress mass and stiffness variations around hemispherical resonator 1110, and rate control drive signal 1230 is used to position the flex wave.
The force applied to hemispherical resonator 1110 by each of the plurality of drive electrodes 1120 is proportional to a direct current (DC) bias voltage Vb maintained on hemispherical resonator 1110. In the case of rate control drive signal 1230 (represented in equation (E2) as Kd), the maximum rate that can be applied to the position of the flex wave is a function of the amplitude of the flex wave as shown by equation (E2):SRCD=Kd*Vb*A*Sin(ωRt+φ)]  (E2).As such, increasing the bias voltage Vb will increase the magnitude of the electrostatic force applied to hemispherical resonator 1110 by each of the plurality of drive electrodes 1120 (i.e., by amplitude drive signal 1210, quadrature control drive signal 1220, and/or rate control drive signal 1230).
Hemispherical resonator 1110 is a high Q oscillator, the Q being, in some instances, approximately 10*106. To control the flex wave of the oscillating hemispherical resonator 1110, all forces applied via the plurality drive electrodes 1120 must be precisely synchronized and phase locked to the natural frequency (ωR) of hemispherical resonator 1110. In current HRG 100 devices, during normal operation the various signals all have a frequency at least approximately equal to the natural frequency ωR of hemispherical resonator 1110. Phase locking is achieved through the use of a phase locked loop 1410 provided by signal processing segment 140. Phase locked loop 1410 tracks the natural frequency ωR of hemispherical resonator 1110 via X readout signal 1310 and Y readout signal 1320 to provide a reference signal for amplitude control loop 1420, quadrature control loop 1430, and rate control loop 1440 to ensure amplitude drive signal 1210, quadrature control drive signal 1220, and rate control drive signal 1230, respectively, have the same frequency as hemispherical resonator 1110.
FIG. 2 is a block diagram of a system 200 including two HRG devices (e.g., HRG 210 and HRG 260). Here, HRG 210 includes it own natural frequency (ωR1) (e.g., 4.1 kHz). As such, any signal (e.g., readout signal 215) output by HRG 210 would include frequency ωR1. Readout signal 215 is filtered by a bandpass filter 220 and transmitted to data processor 230. Any signal representing an angle (Δθ1) output by HRG 210 would include frequency ωR1. Moreover, any signal (e.g., a local oscillator signal) received by HRG 210 would likewise include frequency ωR1 since the frequency of each signal in HRG 210 is controlled by phase locked loop 240. In other words, the frequency (ωLO1) of the local oscillator signal is the same as the natural frequency (ωR1) of HRG 210 (i.e., ωLO1=ωR1).
The operation of HRG 260 is similar to the operation of HRG 210. However, due to the nature of HRGs, the natural frequency (ωR2) of HRG 260 is different from (ωR1). For example, ωR2 may be 4.03 kHz. In other words, ωLO1=ωR1 and ωLO2=ωR2, wherein ωLO1 and ωR1 include a different frequency from ωLO2 and ωR2 such that the signals representing angles Δθ1 and Δθ2 have different frequencies.
Thus, previous systems are complex from a hardware, firmware, and software point of view since each HRG has its own individual natural frequency. Since each HRG has its own individual natural frequency, synchronizing the output signals (e.g., ωR1=4.11 kHz and ωR2=4.07 kHz) from a plurality of HRG devices requires electronics incorporating a set of complex algorithms and computations. Often, the complex algorithms and computations omit important components from one or more signals from the HRG devices since estimating and/or rounding occurs in the algorithms and computations. As such, the resulting signals are not as accurate as they otherwise could be. Notably, these systems become even more complex when each HRG also includes it own accelerometer (not shown) to measure the speed of change in direction for its associated HRG. Thus, there is a need for systems and methods to synchronize output signals from a plurality of HRG devices in a less complex and more accurate manner.